Mesoscale and Nanoscale Physics
Transport characteristics in Hermitian and non-Hermitian Fibonacci rings: A comparative study
Souvik Roy and Santanu K. Maiti
We present an extensive theoretical analysis of transport and circular currents and the associated induced
magnetic fields in Fibonacci rings, explored in both Hermitian and non-Hermitian descriptions, with particular
attention to configurations preserving or breaking PT
symmetry. By engineering physically balanced gain and loss following a Fibonacci sequence, we realize two distinct geometrical configurations in which the ring either preserve or explicitly break PT
symmetry, and further explore complementary realizations obtained by reversing the signs of the on site potentials. Using the non
equilibrium Green's function (NEGF) formalism, we analyze transmission properties and bond current densities to quantify both transport and circulating currents. A comparison with the Hermitian limit establishes a clear baseline, where the ring supports only weak responses upon introducing disorder. In sharp contrast, non-Hermiticity leads to a pronounced amplification of transport and circular currents, and hence of the induced
magnetic field. We further demonstrate that non-Hermitian transport is highly sensitive to gain and loss sign reversal and, in the non-PT-symmetric case, exhibits an unconventional dependence on system size governed by the parity of the Fibonacci sequence and hopping correlations. Remarkably, the current does not decay monotonically with increasing system size, revealing a distinct scaling behavior absent in conventional Hermitian systems. Our results highlight non-Hermitian quasiperiodic rings as versatile platforms for engineering and amplifying current driven
magnetic responses through
symmetry,
topology, and gain-loss design.
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Materials Science
Crystalline-dependent magnon torques in all-sputtered Hf/Cr2O3/ferromagnet heterostructures
Yuchen Pu, Guoyi Shi, Chenhui Zhang et al. (+3 more)
Electron motion in
spin-orbit torque devices inevitably leads to the Joule heating issue.
Magnon torques can potentially circumvent this issue, as it enables the transport of
spin angular momentum in insulating
magnetic materials. In this work, we fabricate a sandwich structure composed of Hf/
antiferromagnetic Cr2O3/ferromagnet and demonstrate that the
magnon torque is strongly dependent on the crystalline structure of Cr2O3.
Magnon torques are stronger when the Neel vector of Cr2O3 aligns parallel to the
spin polarization generated in Hf, while they are suppressed when the Neel vector is perpendicular to the
spin polarization. The
magnon torque efficiency is estimated to be -0.134 using in-plane second harmonic
Hall measurements. Using
magnon torques, we achieve perpendicular magnetization switching of CoFeB, with a critical switching current density of 4.09 x 10^7 A/cm^2. Furthermore, the
spin angular momentum loss due to the insertion of Cr2O3 is found to be lower than that of polycrystalline NiO. Our work highlights the role of antiferromagnet crystalline structures in controlling
magnon torques, broadening the potential applications of
magnon torques.
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All authors Yuchen Pu, Guoyi Shi, Chenhui Zhang, Xinhou Chen, Hanbum Park, Hyunsoo Yang
Materials Science
Structural and magnetic properties of co-sputtered epitaxial Fe-Sn kagome thin films
Callum Brennan-Rich, Sean M. Collins, Stuart Micklethwaite et al. (+5 more)
In recent years the intermetallic
alloys of Fe and Sn have gained significant interest due to a rich variety of
magnetic properties present in these materials. The
crystal Fe3Sn2 is a frustrated ferromagnet, while the crystallographically similar FeSn, which differs only by the stacking sequence of its Fe-containining kagome and stanene layers, is an antiferromagnet. Thin-film growth techniques such as magnetron sputtering allow for these different stoichiometric compositions to be grown through adjustments of the rate of deposition of the individual Fe and Sn sources, while all other conditions remain constant. Here, we report the production of high quality epitaxial thin films of Fe$_3$Sn$_2$ and FeSn on sapphire with a Pt seed layer, as well as a mixed phase containing intergrowths of both
crystals, all of which we have characterized using both X-ray and four-dimensional scanning transmission electron microscopy (4D-STEM) methods. The resulting crystallographic phase content is compared to the results of magnetization measurements, with correspondence between the predicted
ferromagnetic phase content and the resulting magnetization. Further
magnetic properties of these films can then also be compared, leading to the discovery of a unique behavior in the temperature dependent coercivity within highly mixed phase
alloys, a feature that is absent in either pure Fe3Sn2 and FeSn.
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All authors Callum Brennan-Rich, Sean M. Collins, Stuart Micklethwaite, Zabeada Aslam, Trevor Almeida, Stephen McVitie, Rik M. Drummond-Brydson, Christopher H. Marrows
Materials Science
Bio-resorbable magnetic tunnel junctions
Dong-Jun Kim, Beom Jin Kim, Hee-Chang Shin et al. (+7 more)
Magnetic tunnel junctions (MTJs) play a crucial role in spintronic applications, particularly data storage and sensors. Especially as a non-volatile
memory, MTJs have received substantial
attention due to its CMOS compatibility, low power consumption, fast switching speed, and high endurance. In parallel, bio-resorbable electronics have emerged as a promising solution for systems requiring temporary operation and secure data disposal, especially in military, intelligence, and biomedical systems where devices must safely disintegrate under physiological conditions. In this study, we investigate the bio-resorbability of MTJ by analyzing the dissolution behavior of its nanometer-thick constituent layers in phosphate-buffered saline (PBS) solution at pH 7.4, simulating physiological environments. The MTJ structures, composed of bio-resorbable materials, exhibit well-controlled degradation behaviors. Critically, as one of the
ferromagnetic layers dissolves, binary information is irreversibly lost, within 10 hours of immersion. These findings highlight the potential of MTJs not only as high-performance
memory elements but also as secure, transient data storage platforms. The ability to modify the dissolution lifetime by materials and thickness selection offers unique advantages for short-lived implantable devices, paving the way for integrating spintronic functionality into next-generation bioresorbable electronics.
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All authors Dong-Jun Kim, Beom Jin Kim, Hee-Chang Shin, Jeongkeun Kim, Yuchen Pu, Shuhan Yang, Xinhou Chen, Byong-Guk Park, Jong-Hyun Ahn, Hyunsoo Yang
Materials Science
Spin dynamics of excitons and carriers in mixed-cation MA$_{x}$FA$_{1-x}$PbI$_{3}$ perovskite crystals: alloy fluctuations probed by optical orientation
B. F. Gribakin, N. E. Kopteva, D. R. Yakovlev et al. (+5 more)
Optical
spin orientation measured by time-resolved photoluminescence provides a powerful tool to probe the
spin dynamics of
excitons and charge carriers in perovskite
semiconductors. The impact of
alloy fluctuations on the
spin dynamics of mixed-cation \MAFAPI{} perovskite single
crystals is studied here experimentally. The optical orientation is measured under nonresonant excitation for
crystals with $x = 0.1$, $0.4$, and $0.8$ at cryogenic temperatures and compared with data on \MAPI{}
crystals. The high degree of
exciton optical orientation of $75-80$% for $x = 0.1$ and $0.8$ reduces to about 60% for $x = 0.4$. A similar trend is observed for the carrier
spin optical orientation. This behavior is attributed to enhanced scattering of free
excitons and carriers in the
alloys with increased compositional and structural disorder. From the Larmor
spin precession measured from
spin dynamics in an external
magnetic field applied in the Voigt geometry, the electron and hole $g$-factors are evaluated. Their dependence on the band gap energy in \MAFAPI{}
crystals follows the universal trend previously established for lead halide perovskites.
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All authors B. F. Gribakin, N. E. Kopteva, D. R. Yakovlev, I. A. Akimov, I. V. Kalitukha, B. Turedi, M. V. Kovalenko, M. Bayer
Mesoscale and Nanoscale Physics
Quantum geometric scattering of a Dirac particle by a Berry curvature domain wall
Lassaad Mandhour and Frédéric Piéchon
We investigate the scattering of a three-dimensional massless Dirac particle through a domain wall separating two regions with identical energy spectra but distinct Berry
curvature dipoles. We demonstrate that the
quantum geometric mismatch induces partial reflection and transmission despite identical incident and refracted momenta. These results highlight the role of engineered
quantum geometric interfaces as key tools to control Dirac particle scattering.
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Materials Science
Observation of magnon torques mediated by orbital hybridization at the light metal/antiferromagnetic insulator interface
Yuchen Pu, Guoyi Shi, Hua Bai et al. (+5 more)
Magnon torques, which can operate without involving moving electrons, could circumvent the Joule heating issue. In conventional
magnon torque systems, the
spin source layer with strong
spin-orbit coupling is utilized to inject
magnons, and the efficiency is limited by the inherent
spin Hall conductivity of the
spin source layer. In this work, we observe
magnon torques in the Cr/NiO/ferromagnet
heterostructure with the effective
spin Hall conductivity of 2.45x10^5 hbar/(2e{\Omega}m), twice that of the best conventional
magnon torque system. We demonstrate the
magnon-torque-driven switching of a perpendicularly magnetized CoFeB layer at room temperature, with a switching power consumption density of 0.136 mW/{\mu}m^2. We find that the
magnon torque originates from the orbital hybridization and interfacial inversion
symmetry breaking at the Cr/NiO interface. Our findings not only significantly enhance the efficiency of
magnon torques, but also provide key insights into the fundamental mechanisms of
magnon injections.
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All authors Yuchen Pu, Guoyi Shi, Hua Bai, Xinhou Chen, Chenhui Zhang, Zhaohui Li, Mehrdad Elyasi, Hyunsoo Yang
Other Condensed Matter
On the Novel Superfluidity in the Second Layer of $^4$He on Graphite
Jun Usami and Hiroshi Fukuyama
Evidence for a new type of superfluid phase in second-layer $^4$He on graphite has been obtained from simultaneous measurements of torsional-oscillator response and heat-capacity on exactly the same sample down to 30 mK, which resolve substrate-related uncertainties in the previous studies. The new phase hosts both superfluidity and enhanced viscoelasticity, and is stable over a finite density interval, strongly supporting the proposed superfluid
liquid-
crystal hypothesis. A random-Josephson-network analysis shows that the widely reported log-$T$ dependence of the superfluid density is likely due to substrate imperfections.
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Materials Science
Unraveling the effects of anionic vacancies and temperature on mechanical properties of NbC and NbN: Insights from Quantum Mechanical Study
P.W. Muchiri, K. K. Korir, N. W. Makau et al. (+2 more)
Transition metal carbides and nitrides (TMCNs), such as niobium carbide (NbC) and niobium nitride (NbN), are of great technological interest due to their exceptional hardness, high melting points, and thermal stability. While previous studies have focused on their groundstate properties (at 0 K), limited information exists on their mechanical behavior under realistic operational conditions involving elevated temperatures and the presence of defects. In this study, we employ ab initio molecular dynamics (AIMD) simulations to investigate the effects of temperature (300 to 1500 K) and anionic vacancies on the mechanical properties of NbC and NbN in rocksalt (RS), zincblende (ZB), and wurtzite (WZ) structures. The results reveal a nonlinear decrease in elastic constants, bulk, shear, and Youngs
moduli with both increasing temperature and defect concentration. Hardness and toughness analyses, based on Pughs ratio and Poisson ratio, show ductility and brittleness transitions that are sensitive to structure, defect level, and thermal effects. Furthermore, vacancy migration energies computed using the nudged elastic band (NEB) method demonstrate strong structural dependence, with RS exhibiting the highest barriers and WZ the lowest. These findings provide new insights into the defect and temperature interplay in NbC and NbN, offering guidelines for their optimization in high-temperature and wear-resistant applications.
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All authors P.W. Muchiri, K. K. Korir, N. W. Makau, M. O. Atambo, G. O. Amolo
Materials Science
Mechanical control of magnetic exchange and response in GdRu$_2$Si$_2$: A computational study
Sagar Sarkar, Rohit Pathak, Arnob Mukherjee et al. (+3 more)
We present a systematic computational study of the effect of uniaxial strain on the
magnetic properties of GdRu$_2$Si$_2$, a centrosymmetric material known to host a field-induced skyrmion
lattice (SkL). Using first-principles density functional theory, we first demonstrate the pronounced sensitivity of the exchange and anisotropy to specific structural distortions. These DFT-derived interactions are then integrated into a classical
spin model to construct comprehensive
magnetic phase diagrams under both compressive and tensile strain. Our key finding is that compressive strain ($\sim 2\%$) acts as an effective tuning parameter, substantially expanding the stability region of the $\vec Q_{100}$-driven topologically nontrivial phases. This results from the shifts in the critical
magnetic fields and enhancement of the energy scale of the favored
magnetic wave vector. In contrast, tensile strain induces a different
magnetic ground-state by promoting a different
magnetic ordering vector, $\vec Q_{110}$, leading to entirely distinct phase behavior. This work not only provides a quantitative understanding of the structural-
magnetic coupling in GdRu$_2$Si$_2$ but also establishes strain engineering as a powerful approach to control and optimize topologically non-trivial
magnetic phases in centrosymmetric magnets.
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All authors Sagar Sarkar, Rohit Pathak, Arnob Mukherjee, Anna Delin, Olle Eriksson, Vladislav Borisov
Mesoscale and Nanoscale Physics
Quantized heat flow in the Hofstadter butterfly
Aifei Zhang, Gibril Aissani, Quan Dong et al. (+11 more)
When subjected to a strong
magnetic field, electrons on a two-dimensional
lattice acquire a fractal energy spectrum called Hofstadter's butterfly. In addition to its unique recursive structure, the Hofstadter butterfly is intimately linked to non-trivial topological orders, hosting a cascade of ground states characterized by non-zero topological invariants. These states, called Chern
insulators, are usually understood as replicas of the ground states of the
quantum Hall effect, with electrical and thermal conductances that should be quantized, reflecting their topological order. The Hofstadter butterfly is now commonly observed in van-der-Waals
heterostructures-based moiré superlattices. However, its thermal properties, particularly the quantized heat flow expected in the Chern
insulators, have not been investigated, potentially questioning their similarity with standard
quantum Hall states. Here we probe the heat transport properties of the Hofstadter butterfly, obtained in a
graphene~/~hexagonal boron nitride moiré superlattice. We observe a quantized heat flow, uniquely set by the topological invariant, for all investigated states of the Hofstadter butterfly:
quantum Hall states, Chern
insulators, and even
symmetry-broken Chern
insulators emerging from strong electronic interactions. Our work firmly establishes the universality of the quantization of heat transport and its intimate link with
topology.
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All authors Aifei Zhang, Gibril Aissani, Quan Dong, Yong Jin, Kenji Watanabe, Takashi Taniguchi, Carles Altimiras, Patrice Roche, Jean-Marc Berroir, Emmanuel Baudin, Gwendal Fève, Gerbold Ménard, Olivier Maillet, François D. Parmentier
Materials Science
First-principles analysis of in-plane anomalous Hall effect using symmetry-adapted Wannier Hamiltonians and multipole decomposition
Hiroto Saito and Takashi Koretsune
The in-plane anomalous
Hall effect occurs when magnetization lies within the same plane as the electric field and
Hall current, and requires
magnetic point groups lacking rotational or mirror
symmetries. While it is observed in both Weyl semimetals and elemental ferromagnets, the microscopic role of higher-order multipoles remains unclear. Here, we develop a microscopic framework that combines time-reversal-symmetric Wannier functions with a
symmetry-adapted multipole basis to decompose the first-principles Wannier
Hamiltonian into electric,
magnetic,
magnetic toroidal, and electric toroidal multipoles. This approach allows us to rotate the magnetization rank by rank and quantify how each multipole affects the conductivity. Applying this framework to body-centered cubic iron, we find that high-rank
magnetic and
magnetic toroidal multipoles contribute with magnitudes comparable to
magnetic dipoles, while
magnetic toroidal 16-poles act with the opposite sign. Furthermore, based on this multipole analysis, we apply uniaxial strain along the [103] direction to control the dominant multipoles contributing to the conductivity. The strain substantially modifies its angular dependence, demonstrating that multipole-resolved
Hamiltonian engineering and magnetoelastic control serve as practical routes to predict and tune the in-plane anomalous
Hall conductivity in simple ferromagnets.
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Soft Condensed Matter
Joint Optimization of Neural Autoregressors via Scoring rules
Jonas Landsgesell
Non-parametric distributional regression has achieved significant milestones in recent years. Among these, the Tabular Prior-Data Fitted Network (TabPFN) has demonstrated state-of-the-art performance on various
benchmarks. However, a challenge remains in extending these grid-based approaches to a truly multivariate setting. In a naive non-parametric discretization with $N$ bins per dimension, the complexity of an explicit joint grid scales exponentially and the paramer count of the neural networks rise sharply. This scaling is particularly detrimental in low-data regimes, as the final projection layer would require many parameters, leading to severe overfitting and intractability.
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Mesoscale and Nanoscale Physics
Topological Superconductivity in Altermagnetic Heterostructures on a Honeycomb Lattice
George McArdle, Brian Kiraly, Peter Wadley et al. (+1 more)
Altermagnet-
superconductor heterostructures have been shown, in principle, to provide a route towards realising topological superconductivity, and therefore host topologically protected boundary states. In this work we demonstrate that the topological states observed are dependent on the structure of the underlying
lattice. By deriving and analysing a model on a honeycomb
lattice, we demonstrate that the topological phase diagram has a rich structure containing both
chiral edge modes and Majorana corner modes, the latter of which are an indication of higher-order
topology. We analyse the effect of disorder on these states and find that whilst the edge modes are robust to a disordered system, any potential observation of the corner modes may be sensitive to the microscopic details. In particular, we show that vacancies can lead to other low energy bound states that may be difficult to distinguish from the corner modes.
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All authors George McArdle, Brian Kiraly, Peter Wadley, Adam Gammon-Smith
Materials Science
Exact Electrostatic Theory of Fermi-Energy Response at Metallic Interfaces
Théophane Bernhard and Andrea Grisafi
The response of the
Fermi energy to external perturbations governs key physical observables at metallic interfaces. Although this response admits a local formulation in terms of the Fukui function, its evaluation has traditionally been limited by inherent approximations, fundamentally rooted in the difficulty of adding a finite charge in a periodic system. We present an exact resolution to this problem that, within an electronic-structure framework capturing metallic behavior, exploits the screening properties of electronic conductors to compute Fukui functions via a finite electric field. The resulting linear-response theory yields strictly quadratic error scaling of
Fermi-level shifts across representative platinum surfaces, achieving sub-meV accuracy up to fields of 0.1 V/Å. The approach is further validated by reproducing work-function changes under molecular perturbations, and by providing mean-field estimates of electrode potentials that yield capacitance–voltage curves consistent with experiment. Our findings establish a rigorous foundation for a local theory relating electrostatic screening and
Fermi-energy variations at metallic interfaces.
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Materials Science
NiTi Single Crystal Growth by Micro-Pulling-Down Method: Experimental Setup and Material Characterization
Timon Sieweke, Chris Luther, Martin Wortmann et al. (+14 more)
Nickel-titanium that has an austenite to martensite phase transition has been studied extensively in the past as a shape
memory alloy, but a lot remains to be learned from such phase transitions. However, single
crystals are needed for a detailed characterization of the emerging phase transition. In order to produce NiTi single
crystals for research purposes, we have set up a micro-pulling-down ($\mu$PD) apparatus. The $\mu$PD process is a fast and flexible method for the fabrication of small single
crystals. The apparatus is operated in
vacuum. By pulling the
crystal down through a hole in the crucible bottom, it is possible to reduce oxygen contamination, since
oxides float on top of the melt due to their low density. Here we present a detailed characterization of as-grown NiTi
crystals by electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), hot gas extraction method and differential scanning calorimetry (DSC). The characteristics of the phase transition in NiTi are very sensitive to dopants and alloying. The $\mu$PD method facilitates the introduction of different doping elements into the
crystal.
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All authors Timon Sieweke, Chris Luther, Martin Wortmann, Lauritz Schnatmann, Felicitas Werner, Olga Kuschel, Laila Bondzio, Inga Ennen, Judith Bünte, Karsten Rott, Oluwaseyi Oluwabi, Moritz Loewenich, Joachim Wollschläger, Andreas Hütten, Jan Frenzel, Gabi Schierning, Alexander Kunzmann
Statistical Mechanics · Cross-listed from quant-ph
Chaos, thermalization and breakdown of quantum-classical correspondence in a collective many-body system
Ángel L. Corps, Sebastián Gómez, Pavel Stránský et al. (+2 more)
We investigate thermalization and the
quantum-classical correspondence in the collective Bose-Hubbard model, focusing on the four-site case. Our analysis of the classical phase-space structure and its excited-state
quantum phase transitions leads us to three dynamical regimes:
symmetry-breaking low-energy states, an intermediate region where
quantum and classical
equilibrium states markedly disagree, and a high-energy regime with restored correspondence. The observed classical intermittency above the first excited-state
quantum phase transition contrasts with
quantum dynamics, which remains trapped in
symmetry-breaking sectors despite the existence of a classically connected phase. This mismatch originates from the population of imbalance-carrying eigenstates and persists even for relatively large number of particles. Our results reveal unexpectedly slow convergence to the classical limit, signaling robust finite-size effects in collective many-body dynamics.
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All authors Ángel L. Corps, Sebastián Gómez, Pavel Stránský, Armando Relaño, Pavel Cejnar
Soft Condensed Matter
Molecular signatures of pressure-induced phase transitions in a lipid bilayer
Yanna Gautier, Guillaume Stirnemann and Jérôme Hénin
Understanding how lipid
bilayers respond to pressure is essential for interpreting the coupling between membrane
proteins and their native environments. Here, we use all-atom molecular dynamics to examine the pressure-temperature behavior of model membranes composed of DMPC or $\Delta$9-cis-PC. Within the studied range (288-308 K, 1-2000 bar), DMPC undergoes a
liquid–gel transition, while $\Delta$9-cis-PC remains fluid due to unsaturation. The CHARMM36 force field reproduces experimental boundaries with high fidelity: simulated DMPC transitions deviate by only 5-10 K and 100-300 bar, and $\Delta$9-cis-PC exhibits no transition. Hysteresis is modest but most pronounced when starting from low-temperature gels. We identify area per lipid,
bilayer thickness, and acyl-chain gauche fraction as sensitive phase markers; among these, the gauche fraction provides the most robust signature. Simulations indicate an interdigitated gel is the
equilibrium structure under finite-size conditions. However, at low temperature and high pressure, interdigitation decreases, consistent with the experimental lamellar gel phase. This long-lived interdigitation critically impacts standard order parameters, specifically area per lipid and membrane thickness. These results underscore the accuracy of modern force fields and highlight how simulations mechanistically complement experimental studies of pressure-regulated membranes.
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Mesoscale and Nanoscale Physics
Observation of Unconventional Ferroelectricity in Non-Moir'\e Graphene on Hexagonal Boron Nitride Boundaries and Interfaces
Tianyu Zhang, Yueyang Wang, Hongxia Xue et al. (+3 more)
Interfacial interactions in two parallel-stacked hexagonal boron-nitride (hBN) layers facilitate sliding ferroelectricity, enabling novel device functionalities. Additionally, when Bernal or twisted
bilayer graphene is aligned with an hBN layer, unconventional ferroelectric behavior was observed, though its precise origin remains unclear. Here, we propose an alternative approach to engineering such an unconventional ferroelectricity in
graphene-hBN van der Waals (vdW)
heterostructures by creating specific types of hBN boundaries and interfaces. We found that the unconventional ferroelectricity can occur–without the
alignments at
graphene-hBN or hBN-hBN interfaces–when there are hBN edges or interfaces with line defects. By systematically analyzing the gate dependence of mobile and localized charges, we identified key characteristics of localized states that underlie the observed unconventional ferroelectricity, informing future studies. These findings highlight the complexity of the interfacial interactions in
graphene/hBN systems, and demonstrate the potential for defect engineering in vdW
heterostructures.
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All authors Tianyu Zhang, Yueyang Wang, Hongxia Xue, Kenji Watanabe, Takashi Taniguchi, Dong-Keun Ki
Materials Science
Precipitate size evolution in an ultrafine-grained magnesium-manganese alloy
Julian M. Rosalie, Brian R. Pauw and Anton Hohenwarter
Precipitate size
evolution during room temperature high-pressure torsion (HPT) of a Mg-1.35wt.%Mn
alloy was studied using scanning transmission electron microscopy (STEM) and Small-/Wide-angle X-ray scattering (SAXS/WAXS). The volume fraction of the nm-scale $\alpha$-Mn particles increased with applied strain, however small angle X-ray scattering (SAXS) indicated that the majority of manganese remained in solution even after 10 HPT rotations, indicating that the reaction progress is still limited by the diffusivity of Mn. Analysis of the precipitate size distribution determined that the mean particle size did not increase over the course of HPT. This, in combination with the precipitate size distribution suggested that precipitate growth was subject to interfacial rather than diffusional control.
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Materials Science
Symmetry-Driven Unconventional Magnetoelectric Coupling in Perovskite Altermagnets: From Bulk to the Two-Dimensional Limit
Zhou Cui, Ziye Zhu, Xunkai Duan et al. (+4 more)
The emergence of altermagnets establishes a new paradigm for multiferroics. Unlike conventional multiferroics relying on direct magnetoelectric coupling, multiferroic altermagnets host a
crystal-
symmetry-mediated magnetoelectric interaction that is intrinsically more efficient and robust. Among candidate material platforms, layered perovskites are particularly appealing owing to their structural diversity and synthetic versatility. However, magnetoelectric properties at the two-dimensional scale remain largely unexplored, hindering their applicability in miniaturized, highly integrated devices. Here, we systematically investigate the dimensional
evolution of ferroelectric polarization and magnetism in perovskite systems through
symmetry analysis. We demonstrate that altermagnetism can persist in the two-dimensional limit, yet is strongly constrained by the
magnetic configuration-with only C-type
antiferromagnetic order supporting it. Based on mode-decomposition calculations, we further reveal that
symmetry-restricted multimode couplings simultaneously govern ferroelectric polarization and altermagnetic
spin splitting. Finally, combined with first-principles calculations, we propose several strategies to lift the
magnetic-configuration constraint, extending the range of viable altermagnetic systems. These results underscore the critical role of dimensionality in
symmetry-driven magnetoelectric coupling in perovskite altermagnets and pave the way toward next-generation electrically controlled spintronic and multiferroic devices.
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All authors Zhou Cui, Ziye Zhu, Xunkai Duan, Bowen Hao, Xianzhang Chen, Jiayong Zhang, Tong Zhou
Statistical Mechanics
Autonomous Discovery of the Ising Model's Critical Parameters with Reinforcement Learning
Hai Man, Chaobo Wang, Jia-Rui Li et al. (+2 more)
Traditional methods for determining critical parameters are often influenced by human factors. This research introduces a physics-inspired adaptive reinforcement learning framework that enables
agents to autonomously interact with physical environments, simultaneously identifying both the critical temperature and various types of critical exponents in the Ising model with precision. Interestingly, our
algorithm exhibits search behavior reminiscent of phase transitions, efficiently converging to target parameters regardless of initial conditions. Experimental results demonstrate that this method significantly outperforms traditional approaches, particularly in environments with strong perturbations. This study not only incorporates physical concepts into machine learning to enhance
algorithm interpretability but also establishes a new paradigm for scientific exploration, transitioning from manual analysis to autonomous AI discovery.
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All authors Hai Man, Chaobo Wang, Jia-Rui Li, Yuping Tian, Shu-Gang Chen
Quantum Gases
Pattern formation in driven condensates
Kiryang Kwon and Jae-yoon Choi
Spontaneous pattern formation out of homogeneous media is one of the well-understood examples of hydrodynamic instabilities in classical systems, which naturally leads to the question of its manifestation in
quantum fluids. Bose-Einstein condensates (BECs) of atomic gases have been an ideal platform for studying many-body
quantum phenomena, such as superfluidity, and simultaneously providing an opportunity to broaden our understanding of classical hydrodynamics into
quantum systems. In this review, we introduce a range of experimental studies on the pattern formation in
quantum fluids of atomic gases under external driving, including Faraday waves in one and two dimensions, surface patterns, and counterflow instabilities in a mixture of superfluids. The pattern formation in the
quantum system can be understood through the parametric amplification process, where an unstable dynamical mode can be exponentially amplified, similar to classical systems. Remarkably, the governing equations for surface excitations of trapped BECs can be mathematically equivalent to those of shallow water, indicating a universal description of the hydrodynamic instability across classical and
quantum domains. However, the condensates, as superfluids, also possess fundamental
quantum characteristics, such as quantized vorticity and a distinct dissipation channel. These unique features showcase many-body fragmentation under strong modulation and the generation of
vortices in the nonlinear regime, which could offer a pathway to the study of
quantum turbulence. Furthermore, the coexistence of long-range phase coherence and density modulation in driven condensates could provide unexplored features, such as those seen in supersolid-like sound modes, within nonequilibrium settings.
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Strongly Correlated Electrons
Molecular-Orbital Degeneracy Lifting and Suppression of a Flat-Band Metal in a Tetrahedral Cluster System NbSeI
Keita Kojima, Youichi Yamakawa, Ryutaro Okuma et al. (+7 more)
The lifting of degenerate electronic states has been extensively studied in transition metal compounds, where diverse
quantum phenomena arise from the degrees of freedom of d electrons on individual atoms. In contrast, transition metal compounds containing high-
symmetry clusters composed of multiple transition-metal atoms are expected to host a broader range of emergent phenomena due to the
entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct structural responses associated with molecular-orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose cubic average
crystal structure is predicted to host a flat-band metallic state. Below 106 K, NbSeI is a molecular-orbital-ordered nonmagnetic
insulator with orthorhombic
symmetry. Above this temperature, the average structure becomes face-centered cubic without superlattice formation, while local
symmetry breaking associated with pronounced local distortions of the Nb4 tetrahedra lifts the molecular-orbital degeneracy. This local
symmetry breaking occurs without cooperative long-range order and stabilizes a nonmagnetic insulating state instead of the flat-band metallic state predicted for the cubic average structure.
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All authors Keita Kojima, Youichi Yamakawa, Ryutaro Okuma, Shunsuke Kitou, Taishun Manjo, Hayato Takano, Jun-ichi Yamaura, Yusuke Tokunaga, Taka-hisa Arima, Yoshihiko Okamoto
Materials Science
Cross-hatch strain effects on SiGe quantum dots for qubit variability estimation
Luis Fabián Peña, Mitchell I. Brickson, Fabrizio Rovaris et al. (+11 more)
SiGe
heterostructures integrated with Si via virtual substrate (VS) growth are promising hosts for
spin qubits. While VS growth targets plastic relaxation, residual cross-hatch strain inhomogeneity propagates into
heterostructure overgrowth. To quantify strain inhomogeneity's influence on interface structure and
qubit properties, we measure strained-silicon (s-Si)/Si$_{0.7}$Ge$_{0.3}$
heterostructures on 25 wafers processed via standard commercial chemical vapor deposition. Spatially-aligned images of strain (Raman microscopy) and interface structure (atomic force microscopy and cross-sectional scanning transmission electron microscopy) reveal strain-roughness interplay. A strain-driven surface
diffusion model predicts the roughness and its temperature dependence. Measured strains suggest spurious double-dot
qubit detunings of 0.1 meV over 100 nm distances may result. Modeling shows that interface roughness (atomic steps), when convolved with
alloy disorder, only modestly reduces valley splitting (70$\pm$13 vs. 77$\pm$14 $\mu$eV on average). Our findings point to thicker VS buffer layers beneath
heterostructures and lower-temperature growth (T $\le$ 700 $^{\circ}$C) to limit roughening.
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All authors Luis Fabián Peña, Mitchell I. Brickson, Fabrizio Rovaris, J. Houston Dycus, Anthony McDonald, Zachary T. Piontkowski, Joel Benjamin Ruzindana, Adelaide M. Bradicich, Don Bethke, Robin Scott, Thomas E. Beechem, Francesco Montalenti, N. Tobias Jacobson, Ezra Bussmann
Materials Science
Autonomous Probe Microscopy with Robust Bag-of-Features Multi-Objective Bayesian Optimization: Pareto-Front Mapping of Nanoscale Structure-Property Trade-Offs
Kamyar Barakati, Haochen Zhu, C Charlotte Buchanan et al. (+3 more)
Combinatorial materials libraries are an efficient route to generate large families of candidate compositions, but their impact is often limited by the speed and depth of characterization and by the difficulty of extracting actionable structure-property relations from complex characterization data. Here we develop an autonomous scanning probe microscopy (SPM) framework that integrates automated atomic force and
magnetic force microscopy (AFM/MFM) to rapidly explore
magnetic and structural properties across combinatorial spread libraries. To enable automated exploration of systems without a clear optimization target, we introduce a combination of a static physics-informed bag-of-features (BoF) representation of measured surface morphology and
magnetic structure with multi-objective
Bayesian optimization (MOBO) to discover the relative significance and robustness of features. The resulting closed-loop workflow selectively samples the compositional gradient and reconstructs feature landscapes consistent with dense grid "ground truth" measurements. The resulting Pareto structure reveals where multiple nanoscale objectives are simultaneously optimized, where trade-offs between roughness, coherence, and
magnetic contrast are unavoidable, and how families of compositions cluster into distinct functional regimes, thereby turning multi-feature imaging data into interpretable maps of competing structure-property trends. While demonstrated for Au-Co-Ni and AFM/MFM, the approach is general and can be extended to other combinatorial systems, imaging modalities, and feature sets, illustrating how feature-based MOBO and autonomous SPM can transform microscopy images from static data products into active feedback for real-time, multi-objective materials discovery.
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All authors Kamyar Barakati, Haochen Zhu, C Charlotte Buchanan, Dustin A Gilbert, Philip Rack, Sergei V. Kalinin
Materials Science
Unusually high phonon thermal conductivity in the Weyl semimetal TaP: A comparative study with TaAs
Xianyong Ding, Xin Jin, Dengfeng Li et al. (+5 more)
In many metals, thermal transport is often dominated by electrons, although the
lattice contribution can remain appreciable depending on the material. Here, through rigorous first-principles calculations, we uncover a
phonon-dominated thermal transport regime in the Weyl semimetals TaAs and TaP. Remarkably, despite its metallic character, TaP exhibits an exceptionally high
phonon thermal conductivity ($\kappa_{\rm ph}$) of 162 Wm$^{-1}$K$^{-1}$ at room temperature, surpassing its electronic counterpart by nearly an order of magnitude. This anomalously high $\kappa_{\rm ph}$ arises from the combined effects of Weyl electronic structure, acoustic
phonon bunching, and a large low-high frequency
phonon gap, which together suppress
phonon-electron and three-
phonon scattering. The linearly dispersing bands near the
Fermi level yield a low electronic density of states, limiting both electrical conductivity and electronic thermal transport in these compounds. By further surveying a broad range of topological semimetals, we show that the prominence of
phonon thermal transport is a universal characteristic of this material class. Our work provides deeper insight into thermal transport mechanisms in topological semimetals and broadens the scope for discovering metals with high thermal conductivity.
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All authors Xianyong Ding, Xin Jin, Dengfeng Li, Jing Fan, Peng Yu, Xiaoyuan Zhou, Xiaolong Yang, Rui Wang
Strongly Correlated Electrons · Cross-listed from math-ph
An Operator-Algebraic Framework for Anyons and Defects in Quantum Spin Systems
Siddharth Vadnerkar
In this dissertation, we detail an operator algebraic approach to studying topological order in the infinite volume setting. We give a thorough and self-contained review of the DHR-style approach on
quantum spin systems, which builds a category $\mathrm{\textbf{DHR}}$ of anyon sectors starting from microscopic
lattice spin systems. In general, this category has the structure of a braided $\mathrm{C}^*$-tensor category. We will verify in full detail that $\mathrm{\textbf{DHR}}$ is the expected category in Kitaev's
Quantum Double model, a paradigmatic model for studying topological order on the
lattice. We will then extend the DHR-style analysis to systems in the presence of a global on-site
symmetry, and introduce a category of
symmetry defects, $G\mathsf{Sec}$, and show that it has the structure of a $G$-crossed braided $\mathrm{C}^*$-tensor category.
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Statistical Mechanics · Cross-listed from quant-ph
Emergence of the 2nd Law in an Exactly Solvable Model of a Quantum Wire
Marco A. Jimenez-Valencia and Charles A. Stafford
As remarked by Boltzmann, the Second Law of Thermodynamics is notable for the fact that it is readily proved using elementary statistical arguments, but becomes harder and harder to verify the more precise the microscopic description of a system. In this article, we investigate one particular realization of the 2nd Law, namely Joule heating in a wire under electrical bias. We analyze the production of
entropy in an exactly solvable model of a
quantum wire wherein the conserved flow of
entropy under
unitary quantum evolution is taken into account using an exact formula for the
entropy current of a system of independent
quantum particles. In this exact microscopic description of the
quantum dynamics, the
entropy production due to Joule heating does not arise automatically. Instead, we show that the expected
entropy production is realized in the limit of a large number of local measurements by a series of floating thermoelectric probes along the length of the wire, which inject
entropy into the system as a result of the information obtained via their continuous measurements of the system. The decoherence resulting from inelastic processes introduced by the local measurements is essential to the phenomenon of
entropy production due to Joule heating, and would be expected to arise due to inelastic scattering in real systems of interacting particles.
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Strongly Correlated Electrons
Revival of Strain Susceptibilities: Magnetostrictive Coefficient and Thermal-Expansion Coefficient
Yisheng Chai
In thermodynamics, volume is an essential extensive variable. Strain-line, area, or volume change-therefore offers a direct window into correlated
quantum matter: tiny length changes {\Delta}L track how the
lattice responds when state variables such as
magnetic field H and/or temperature T are varied, revealing phases, transitions, and dynamics. Direct, high-precision strain measurements are already difficult; their susceptibilities are harder still. Very recently, several direct techniques have made vital progress on two key quantities: the magnetostrictive coefficient d{\lambda}/dH (often denoted qijk or dij in the magnetostriction literatures), and the linear thermal-expansion coefficient {\alpha}= d{\lambda}/dT. Considering these two strain susceptibilities together-they are fundamental and complementary-clarifies why these thermodynamic properties merit renewed
attention.
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